1. Field of Invention
This invention relates to acoustic analysis of plastic composition materials such as graphite and fiber reinforced epoxy materials. In particular, the invention relates to a method and apparatus for determining and controlling potential dimensional stability, including relaxation stress potential, of composite materials through the use of acoustic sensing techniques during solidification and subsequent thermal loading.
Advanced composite materials, primarily graphite and aramid fiber reinforced epoxy resin materials, are attractive to a number of applications due to their high specific stiffness, high strength and very low coefficients of thermal expansion. Among the applications are spacecraft antennas and microwave components, which impose stringent demands on dimensional stability over a broad range of operating temperature and stress conditions. However, such composite materials are heterogeneous as well as anisotropic. Consequently, thermal treatment, including thermal loading during and subsequent to formation of the solid state of the material, induces internal residual stresses. The stresses are subject to relaxation with time, which result in dimensional modification, the extent of which may be unacceptable in certain critical applications, such as spacecraft antenna systems. Therefore, great interest exists in predicting conditions for stress relief and in determining the effect of preconditioning thermal treatments on dimensional stability of composite materials.
2. Description of the Prior Art
The following patents were uncovered in a search of the public records of the U.S. Patent and Trademark Office respecting the subject invention:
Garst, U.S. Pat. No. 3,985,712; PA1 Smith, U.S. Pat. No. 4,022,555; and U.S. Pat. No. 3,819,915; PA1 Dembiak et al., U.S. Pat. No. Re. 29,886; PA1 Le Dall, U.S. Pat. No. 4,275,448; and U.S. Pat. No. 4,237,538.
In addition, a paper has been coauthored by the inventor of the present invention after the present invention was made. The paper was presented at the Air Force/Defense Advanced Research Project Agency Review of Progress in Quantitative NDE at the University of Colorado, Boulder on Aug. 3, 1981. The paper has been published in the Proceedings of the Review in about October 1982.
The Garst patent discloses a method and apparatus for preparing polyester resins in which the resin is monitored and controlled by a computer which takes periodic temperature readings and compares the temperature readings with an empirically determined temperature/time curve stored in the computer. Heat is then either added or removed from the reactor, vessel so that reactor temperature is made to coincide with the temperature/time curve for the empirically derived ideal material state. This patent is representative of inventions which rely on temperature/time characteristics as an indirect means for analyzing and controlling the integrity of materials during formation of a solid or semisolid state. However, no suggestion is made on how to analyze or control stress-related charcteristics such as distributed defects and residual stress relief characteristics.
The Smith patents describe inventions relating to the control of vulcanization processes in which a reaction rate is assumed based on a monitored temperature. The apparatus according to the Smith patents as well as the method embodied in the apparatus fail to teach or suggest direct analysis or control of stress-related characteristics of the material being formed.
The Dembiak et al. patent discloses methods of an apparatus for controlling the thickness of an annular plastic extrusion wherein an ultrasonic reflected signal is used in conjunction with an appropriate positioning or function control device to control the amount of material removed.
The Le Dall patents disclose a digitally controlled process directed to the treatment of resins and are representative of the state of the art of resin process control. There is no suggestion of any mechanism for controlling stress-related characteristics.